Whole House Generator Sizing Calculator
Whole-house generator sizing for residential standby power ensures all home systems remain operational during utility outages, which according to the EIA averaged 5.8 hours per US...
Formula
Source: NEC Article 220 (Load Calculations), NEC Article 702 (Optional Standby Systems), NFPA 110 | Last reviewed: July 26, 2026
Examples
1 kW
= 30.36 kW
- ac_tons = 4
- electric_range = 1
- electric_dryer = 1
- well_pump = 1
- hot_tub = 0
- ev_charger = 1
- margin = 20
4-ton AC, 2 major appliances, well pump, EV charger -> ~30 kW generator recommended
1 kW
= 8.88 kW
- ac_tons = 2
- electric_range = 0
- electric_dryer = 0
- well_pump = 0
- hot_tub = 0
- ev_charger = 0
- margin = 20
Essential loads only (~9 kW) -> select 10 kW generator
1 kW
= 33.1 kW
- ac_tons = 5
- electric_range = 1
- electric_dryer = 1
- well_pump = 2
- hot_tub = 1
- ev_charger = 1
- margin = 25
Large home with all-electric appliances -> 33 kW (or 26 kW with soft starts)
Quick Reference Table
| Home Profile | Recommended Generator Size | Typical Installed Cost | Notes |
|---|---|---|---|
| Essential circuits (no AC) | 7-10 kW | $5,000-8,000 | Fridge, lights, furnace blower, sump/well pump, outlets |
| Small home (<1,500 sq ft), 2-ton AC, soft start | 16-18 kW | $7,000-10,000 | Air-cooled, adequate for most small homes |
| Medium home (1,500-2,500 sq ft), 3-4 ton AC, soft start | 20-22 kW | $9,000-12,000 | Most popular configuration; air-cooled |
| Large home (2,500-3,500 sq ft), 4-5 ton AC, soft start | 24-26 kW | $11,000-15,000 | Largest air-cooled units; may need gas meter upgrade |
| All-electric home, 2 ACs or heat pumps, EV charger | 30-38 kW | $16,000-22,000 | Liquid-cooled required; load management recommended |
| Luxury/estate home, multiple ACs, pool, guest house | 45-60 kW | $22,000-35,000 | Liquid-cooled; may require multiple units paralleled |
| Appliance | Running Watts | Starting Surge Watts | Notes |
|---|---|---|---|
| Refrigerator/Freezer | 150-800 | 1,000-2,000 | Energy Star models on lower end; 20+ year old on upper end |
| Gas Furnace Blower (1/2 HP) | 400-800 | 1,200-2,400 | ECM motors draw less; PSC motors draw more |
| Central AC (per ton) | 1,200-1,800 | 4,000-7,000 | With soft start: starting surge ~2x running |
| Well Pump (1 HP) | 1,200-1,500 | 5,000-7,000 | Deep well pumps (>200 ft) may be higher |
| Electric Water Heater | 4,500 | 4,500 | Resistive load -- no starting surge |
| Electric Range (large burner) | 2,500-3,500 | 2,500-3,500 | Per element; oven adds 3-5 kW |
| Electric Dryer | 4,500-5,400 | 4,500-5,400 | Motor + heating element; element cycles |
| EV Charger (Level 2, 32A) | 7,700 | 7,700 | Adjustable -- set to lower current on generator |
| Hot Tub Heater | 5,500-7,000 | 5,500-7,000 | Plus 1.5-3 HP pump motor |
| Microwave Oven | 1,000-1,500 | 1,000-1,500 | Short duty cycle; rarely coincident with range |
Where is this used?
(1) NEC Article 220 load calculation: The definitive regulatory method -- the electrician calculates the total connected load, applies demand factors per NEC 220 Part III (for dwelling units, including 3VA/sq ft for general lighting and receptacles, 1,500VA per small appliance circuit, and nameplate ratings for fixed appliances), and selects a generator with capacity exceeding the calculated load.
For generators with automatic load management, the managed load calculation per NEC 702.4(B)(2) applies.
(2) Manufacturer sizing tools: Generac Power Design Pro, Kohler SiteSizer, and Cummins PowerSuite provide residential-specific sizing modules that incorporate each manufacturer's load management module capabilities, generator ratings, and transfer switch options.
These tools generate permit-ready documentation.
(3) Essential vs whole-house decision: Many homeowners initially request 'whole-house' coverage but, after cost comparison, select 'essential circuits only' (7-10 kW) or 'managed whole-house' (20-26 kW) -- the price difference is $5,000-10,000 installed.
The decision typically depends on outage frequency (rural areas with overhead lines experience 3-5x more outages than urban areas with underground utilities), medical equipment needs (CPAP, oxygen concentrator, home dialysis), food storage (chest freezers with $500-1,000 of meat), home-based business continuity, and budget.
(4) Natural gas vs propane fuel decision: 65-70% of residential standby generators in the US are fueled by natural gas (connected to the utility gas line -- no refueling needed, but the gas utility may be interrupted during earthquakes or major disasters).
The remaining 30-35% use propane from an on-site tank (250-1,000 gallon), which stores indefinitely without degradation.
Propane generators in rural areas without natural gas service require a tank large enough to supply the generator for the desired outage duration: a 22 kW generator burns approximately 2.5-3.5 gallons/hour at full load, so a 500-gallon tank provides 140-200 hours (6-8 days) of continuous operation at full load.
(5) Generator placement and code compliance: NEC and manufacturer requirements dictate minimum clearances: 5 feet from windows, doors, and any building opening (to prevent CO intrusion); 3 feet from the building wall on the intake/exhaust sides; and 18 inches from the building on the non-service sides.
Local zoning may impose additional setback requirements and noise limits (typically 60-70 dBA at 23 feet for air-cooled units; liquid-cooled units with sound-attenuated enclosures achieve 55-65 dBA).
CO detectors are required by code in all sleeping areas when a generator is installed.
(6) Transfer switch options: Automatic Transfer Switch (ATS) -- senses utility loss, starts the generator, and transfers the load automatically (typically within 10-30 seconds, adjustable).
Service-entrance rated ATS eliminates the need for a separate main disconnect, simplifying installation.
Manual transfer switch -- lower cost ($200-400 vs $500-1,200 for ATS) but requires the homeowner to manually start the generator and transfer loads -- not suitable for unattended operation.
ATS with load management -- the same ATS can control up to 4-8 load management modules (LMMs), each capable of shedding a single 240V load (AC, dryer, EV charger, hot tub) when the generator approaches capacity.
(7) Generator exercise and maintenance: Per NFPA 110 (adopted by reference in most building codes), residential generators must be exercised under load at least monthly.
Most modern generators have an automatic exercise cycle (typically every other week for 12-20 minutes at a pre-set time).
The exercise burns off moisture, lubricates seals, charges the starting battery, and verifies readiness.
Homeowners should check the generator's oil level, coolant level (liquid-cooled units), battery electrolyte level, and air filter condition quarterly, and schedule annual professional maintenance including oil change, spark plug replacement, and transfer switch inspection.
Real-World Usage Scenarios
Suburban Family Home Whole-House Generator Installation
A 2,800 sq ft colonial home in suburban New Jersey with 4-ton central AC, gas furnace (800W blower), gas range, electric dryer, 1 HP well pump, and one EV charger. The homeowner wants whole-house coverage during outages (area averages 3-4 outages/year, each 6-24 hours). With soft start on the AC: base load 3 kW + AC running 3.2 kW + dryer 2.5 kW + well pump 1.5 kW + EV charger 2.5 kW = 12.7 kW coincident. With 20% margin: 15.2 kW. The dealer recommends a 22 kW Generac Guardian air-cooled unit with an automatic transfer switch and two load management modules (controlling the EV charger and the dryer). Installed cost: $10,500 including gas connection, concrete pad, and electrical permit. The load management ensures the generator never exceeds 90% of its rated capacity by shedding the dryer and EV charger when necessary. During an outage, the home operates normally with all lights, refrigeration, well pump, and AC -- the dryer simply waits until the AC cycles off.
All-Electric Home in Hurricane-Prone Florida
A 3,200 sq ft single-story home in Naples, FL, with two 3-ton heat pumps (no gas service available), electric water heater (4.5 kW), electric range, electric dryer, pool pump (1.5 kW), and an EV charger. The area experiences hurricane-related outages that can last 5-10 days. Without load management, the connected load reaches approximately 38 kW, requiring an expensive liquid-cooled generator. The electrical contractor designs a managed system: a 26 kW Kohler 26RCA liquid-cooled generator with automatic load management controlling both heat pumps (only one runs at a time during generator operation), the EV charger (locked out during generator mode), the electric dryer, and the water heater (interlocked with the heat pump backup). The essential loads (lighting, refrigeration, well pump, one heat pump cycling, electronics) total approximately 12 kW continuous. The second heat pump and water heater cycle on a priority basis. Installed cost: $18,500 including a 500-gallon propane tank (natural gas is not available). The propane supply provides approximately 7 days of continuous operation at 60% average load.
Rural Farmhouse with Outbuildings and Livestock Needs
A 2,200 sq ft farmhouse in rural Wisconsin with a 2-ton AC, gas furnace, electric water heater, electric range, electric dryer, 2 HP well pump (serving the house and barn), barn heaters (two 1.5 kW electric space heaters for livestock water tanks), and barn lighting (1 kW). Winter outages of 24-72 hours are common. The farm's critical path: well pump failure means no water for the household or 40 head of cattle. The electrician sizes for the worst-case winter scenario: base load 3 kW + gas furnace blower 0.8 kW + well pump 3 kW running (2 HP) + water heater 4.5 kW (intermittent) + barn heaters 3 kW = 14.3 kW coincident continuous. With 25% margin: 17.9 kW. The well pump's starting surge (2 HP x 0.7457 x 6 = 8.9 kW starting allowance) adds to the equation: (14.3 + 8.9) x 1.25 = 29 kW. Selected: 30 kW liquid-cooled diesel generator with a 200-gallon fuel tank providing 48 hours of operation at full load. The diesel fuel is treated with biocide and stabilizer, and the generator is exercised monthly under a 15 kW load bank. Installed cost: $22,000. The ability to maintain livestock watering during a winter outage justified the investment -- the value of the cattle herd ($60,000+) dwarfs the generator cost.
Common Mistakes to Avoid
Ignoring the NEC load calculation and using rules of thumb
Many homeowners and even some contractors 'eyeball' generator sizing: 'a 2,500 sq ft house needs 20 kW.' This rule-of-thumb approach ignores two critical variables: AC tonnage and the presence of electric vs gas appliances. A 2,500 sq ft house with 5 tons of AC, electric range, and electric dryer may need 26-30 kW, while the same house with gas range, gas dryer, and a 3-ton AC with soft start may only need 16-18 kW. The NEC Article 220 load calculation (or the simpler optional method for dwelling units) is the only legally defensible sizing method, and most building departments require it as part of the electrical permit for generator installation. A generator sized by rule of thumb that proves inadequate during an outage is both a safety hazard and a liability for the installer.
Overlooking the well pump starting surge
Well pumps, especially deep submersible pumps (200-400 ft depth), have high starting currents and are frequently the overlooked load that trips an undersized generator. A 2 HP, 240V submersible pump draws approximately 12A running (2.9 kW) but 55-70A locked-rotor (13-17 kW momentarily). If the generator is sized for the running load only, the pump may either fail to start -- leaving the home without water -- or cause the generator to trip offline from the starting surge. The problem is worse with older pumps that have higher locked-rotor current, and in cold weather when water in the pipe column increases starting torque requirements. Always verify well pump LRA from the motor nameplate and include the starting surge in the sizing calculation. A 20 kW generator that can run a 4-ton AC plus the house base load may still trip when the well pump starts while the AC compressor is running.
Assuming natural gas will always be available during an outage
Approximately 65-70% of residential standby generators in the US are fueled by natural gas, and a common selling point is 'never refuel -- it runs on your gas line.' While natural gas infrastructure is highly reliable, it is not immune to outage -- earthquakes can rupture gas mains (2014 Napa quake shut off gas to 30,000+ customers), floods can inundate regulator stations, and in rare cases, utility operators may shut off gas proactively during extreme wildfire risk conditions (as PG&E has done in California during high-wind events). For homeowners in seismic zones (California, Pacific Northwest, Alaska) or wildfire-prone areas, a dual-fuel generator (natural gas + propane) or a dedicated propane generator with an on-site tank provides fuel security. If selecting natural gas, confirm with the local gas utility that the service line and meter have sufficient capacity to supply both the generator at full load (22 kW generator consumes approximately 300-350 CFH or 3-3.5 therms/hour) and the home's other gas appliances simultaneously -- some older residential gas services (250 CFH meters) are undersized for a 22+ kW generator plus a tankless water heater plus a gas range plus a gas dryer, potentially requiring a meter upgrade at $500-1,500 cost.
Industry Standards Referenced
Frequently Asked Questions
What size whole house generator do I need?
Essential circuits only: 7-10 kW (refrigerator, lights, furnace blower, sump pump, well pump, internet/router, a few receptacles). Whole house without AC or with small AC and soft start: 10-16 kW. Whole house with AC using soft start: 20-22 kW. Whole house with AC (no soft start): 26-30+ kW. Large luxury home with multiple ACs, pool equipment, and EV chargers: 30-48 kW with load management. The exact requirement depends on your specific appliances, their starting characteristics, whether you have gas or electric cooking/drying/water heating, and whether load management is used. An NEC Article 220 load calculation by a licensed electrician is the only code-compliant method and is required for permitting in virtually all jurisdictions. Never rely solely on a rule-of-thumb or online calculator for purchase -- always get a professional on-site assessment.
What is the difference between air-cooled and liquid-cooled generators?
Air-cooled generators (typically 7-26 kW): Use fan-cooled engines similar to lawn tractor engines cooled by airflow over finned cylinders and an oil cooler. Less expensive to purchase ($5,000-10,000 installed for a complete system), smaller physical footprint, and adequate for most residential applications where outages are measured in hours to a few days. Engine life typically 2,000-4,000 hours. Liquid-cooled generators (22-150+ kW): Use automotive-style water-cooled engines with a radiator, coolant pump, and thermostat -- identical to car engine cooling systems. More durable for frequent and prolonged outages, quieter (water jacket absorbs noise), and capable of much higher power output. Engine life typically 10,000-30,000 hours. More expensive to purchase ($12,000-25,000+ installed) and maintain (coolant changes, water pump replacement). For most residential whole-house applications, a 22-26 kW air-cooled unit is the sweet spot of cost vs capability. Liquid-cooled becomes necessary for homes exceeding approximately 30 kW demand or for prime-power-type usage patterns (frequent multi-day outages).
Do I need a soft start kit for my AC?
If your generator is 20-24 kW and your AC is 3-5 tons, a soft start kit is highly recommended and often essential. Without a soft start, the AC compressor's locked-rotor current (typically 60-90A at 240V = 14-22 kW momentary demand) may overload a 20 kW generator, causing the compressor to stall and the generator breaker to trip. A soft start kit ($300-500 installed for popular brands like Micro-Air EasyStart) limits the starting current inrush to approximately 2x FLA instead of 5-6x, reducing the starting surge from approximately 18 kW to approximately 7 kW for a 4-ton AC -- well within a 20 kW generator's capability. This $500 add-on can reduce the required generator size by 8-12 kW, saving $3,000-6,000 on the generator purchase. The soft start also extends compressor life by reducing mechanical stress during starting. For homeowners with a 22-26 kW generator and a 3.5-5 ton AC, a soft start is the single most cost-effective way to ensure reliable generator operation.
How much does a whole-house generator cost installed?
Total installed cost varies significantly by region, generator size, fuel type, and site conditions. Air-cooled 20-22 kW: $8,000-12,000 including generator, automatic transfer switch, concrete pad, gas connection, electrical wiring, permits, and labor. Air-cooled 24-26 kW: $10,000-15,000. Liquid-cooled 30-38 kW: $16,000-22,000. Liquid-cooled 45-60 kW: $22,000-35,000. Additional costs that may apply: propane tank installation ($1,500-4,000 for 250-1,000 gallon, above or below ground), gas meter upgrade if existing meter is undersized ($500-1,500), electrical service upgrade if the home panel cannot accommodate the transfer switch ($2,000-5,000 for a 200A to 400A upgrade), tree removal or site grading ($500-2,000), and sound-attenuated enclosure upgrades ($1,000-3,000). Federal, state, or utility incentives are rare for residential generators (unlike heat pumps or solar). The total investment for a typical 22 kW whole-house generator installation in a standard suburban home is $10,000-13,000. Annual maintenance (oil change, filter, inspection) costs $200-400 if performed by a dealer, or $50-100 in materials for DIY maintenance.
Can a portable generator power my whole house instead of a standby generator?
A large portable generator (10-15 kW) connected through a manual transfer switch or generator interlock kit can power most essential circuits and potentially some 240V loads, but cannot match the convenience, capacity, or safety of a permanently installed standby generator. Key differences: Portables must be manually started, connected, and refueled (typically every 8-12 hours -- requiring gasoline storage of 20-30 gallons for a 3-day outage). They produce unregulated power quality that may damage sensitive electronics. They are louder (75-85 dBA vs 60-70 dBA for enclosed standby units). They lack automatic transfer -- the homeowner must be present to start and connect the generator. For occasional short outages (a few hours, a few times per year), a portable generator with a manual transfer switch ($1,000-3,000 total) is a reasonable solution. For frequent or extended outages, unattended operation (vacation home, elderly residents), or whole-house coverage including central AC, a permanently installed standby generator is the appropriate choice. Many homeowners start with a portable and upgrade to a standby after experiencing a multi-day winter outage.
Reviewed for accuracy
Reviewed against NEC 2023 Articles 220 and 702, NFPA 110-2022, and Generac/Kohler/Cummins residential product specifications · Last reviewed: July 26, 2026
All calculations are for reference only. Always verify with manufacturer data and a qualified engineer for critical applications. Learn about our editorial process.